Many hydrocarbon processes comprise a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to convert the hydrocarbons down to desirable products. During this process, the catalyst tends to accumulate coke thereon, which is burned off in the regenerator.
In one such process, fluid catalytic cracking (FCC), the heat of combustion in the regenerator typically produces flue gas at temperatures of 677° to 788° C. (1250° to 1450° F.) and at a pressure range of 138 to 276 kPa (20 to 40 psig). Although the pressure is relatively low, the extremely high temperature, high volume of flue gas from the regenerator contains sufficient energy to warrant economic recovery.
To recover energy from a flue gas stream, flue gas may be fed to a power recovery unit. A power recovery train may include several devices, such as an expander turbine, a generator, an air blower, a gear reducer, and a let-down steam turbine. The energy of the flue gas is transferred through blades of the expander to a rotor coupled either to a main air blower, to produce combustion air for the regenerator, and/or to a generator to produce electrical power. Because of the pressure drop of 138 to 207 kPa (20 to 30 psi) across the expander turbine, the flue gas typically discharges with a temperature drop of approximately 125° to 167° C. (225° to 300° F.). The flue gas may be run to a flue gas steam generator for further energy recovery and cooling the flue gas.
A typical flue gas steam generator for an FCC unit contains three coils, namely an economizer for preheating boiler feed water, an evaporator for generating high pressure steam and a super heater for heating the high pressure steam into superheated steam. After removing fines, the cooled flue gas may then be discharged into the stack.
Lower flue gas discharge temperatures indicate greater heat recovery from the flue gas. However, the minimum flue gas discharge temperature is limited by the sulfuric acid dew point. A sufficient temperature approach above the dew point is required for flue gas to avoid sulfuric acid condensation in the stack which can lead to sulfuric acid precipitation in the vicinity of the stack.
In an FCC unit, hydrocarbon feed such as vacuum gas oil (VGO) is typically preheated by indirect heat exchange with a slurry oil pump around from the main fractionation column. The main fractionation column receives hot FCC products. The feed heat exchange with the slurry oil pump around helps to cool FCC products.
There is need for improved recovery of heat from catalyst regenerator flue gas.